thanks moodle!
I did reupload the theme (before I started all over again) and it still had that error. Then I decided to start all over again. Now, I haven t done anything and it says the “stylesheet is missing” at the default TwentyTen theme.
And yes, i did delete the database but I exported the files before. That is not a big deal. My problem is the missing default.css because it means I cannot edit the theme the way I want to. I have no idea where it is – and because it is missing at the theme TwentyTen as well I thought it might be a problem with my host. It doesn t allow .htaccess.